Carbon molecular sieve membranes are an example of microporous membranes (pore size less than 20 Angstroms), which separate gas molecules on the basis of their different diameters (molecular sieving). The mechanism for such membrane transport is based on a combination of adsorption within and activated diffusion along the pore length. Studies of gas permeation through microporous membranes show selectivities much greater than would be expected from Knudsen diffusion--the predominant bulk process in micropores. Therefore, the process can be seen to occur predominantly through hopping between adsorption sites. The permeation rate will be proportional to the product of the local gradient of the concentration of gas molecules adsorbed on the pore and the effective diffusion coefficient (D) at that point in the pore.
The temperature dependence of the permeability in membranes will depend on the temperature dependence of the activated diffusion coefficient D, and that of the extent of adsorption. While adsorption isotherms show that the extent of adsorption for most gases on most materials will increase with a drop in temperature, the activated diffusion coefficient often drops much more drastically, leading to a net drop in permeability with the drop in temperature. This is particularly the case for glassy polymers where the process for activated transport involves some movement of polymer segments to allow movement of gas molecules through interconnected free volumes. Even though some polymer membranes are known to show an increase in selectivity with decrease in temperatures, they suffer a disastrous loss of permeability even for the faster member of a gas pair as the membrane approaches the temperature where all polymer motion is frozen out. Such polymer membranes are therefore of limited applications at low temperatures. This is illustrated by FIG. 1 which shows that for polytrimethylsilyl propyne (PTMSP), one of the more state of the art membranes, the permeability drops drastically below -20.degree. C.